Colour-formers are chemicals that can exist in two or more coloured states that can include a colourless state. The transition between coloured states is achieved by a change in the environment of the colour-former (CF). This change can, for example, be a change in pressure, temperature, light intensity or solvent. In each of these cases, the CF requires custom formulation with other excipients to respond to each specific environmental change. In general, different excipients are required to translate different environmental stimuli into a change in the coloured state of the CF; thus, the same CF can be employed in a range of colour-indicating applications for a variety of environmental changes.
For example, regarding the measurement of instantaneous pressure, Fuji Photo Film Co., Ltd. (“Fuji”) produced a single-use pressure recording film, originally named Fuji Prescale Film. Fuji produced these pressure recording films in a range of grades (0.2-130 MPa). The basis of this product (for pressures in the range 0.2-50 MPa) is a pair of films; the first (named A-film) is coated with a layer of microspheres containing a colour-forming dye in its colourless state, the second (named C-film) is coated with a layer of a colour developer (CD). The colour developer converts the CF from a one coloured state (typically colourless) to another (typically coloured). The two films are stored separately to avoid accidental damage prior to use. During use, the two films are brought gently together with coated faces in contact. The application of a suitable localised force to the bilayer ruptures the microspheres containing the colour forming dye. The released dye contacts the colour developer and results in a localised strong colour that is visible through the transparent back of the film. This prior art is described further in UK Patent No. 1426641 and U.S. Pat. No. 4,447,075.
The pressure recording film described above is intended for the measurement of both momentary and continuous pressure distributions. According to user instructions, momentary pressure is recorded for 10 seconds and continuous pressure is recorded for 4 minutes. It is inherent in the construction of this pressure measurement system that the recording of cumulative, variable applied pressures over several hours or days is not possible.
Furthermore, the pressure recording film described above has a minimum pressure threshold of 0.2 MPa (1500 mmHg) and this threshold is significantly higher than the pressure range that is relevant for at least some applications of embodiments of the present invention as described below.
The low pressure threshold of existing electronic and non-electronic pressure-responsive arrays is dependent upon the mechanical properties of the materials from which these arrays are constructed.
An aim of certain embodiments of the present invention is to provide apparatus that enable the measurement of cumulative pressure.
An aim of certain embodiments of the present invention is to record pressures accumulated in the range of minutes to days within a total duration of hours to years.
An aim of certain embodiments of the present invention is to provide apparatus that enable the measurement of cumulative pressure with a response threshold in the range 0-1000 mmHg, e.g. 0-200 mmHg.
Certain embodiments of the present invention may have broad application in a range of fields requiring the passive recording of accumulated pressure including structural engineering, medical devices and sporting goods. For some of these applications, it may be desirable for the technology to be both robust and flexible.